Fatigue testing shake table apparatus



V. D. COOMBS FATIGUE TESTING SHAKE TABLE rrARAfrug I 2 Sheets-Sheet 1Aug. 9, 1966 Original Filed March 30, 1962 INVENTOR. VERNER D. COOI'IBSATTORNEY Aug. 9, 1966 v. D. COOMBS 3,24,86

' FATIGUE TESTING SHAKE TABLE APPARATUS Original Filed March 30, 1962 2Sheets-Sheet 2 IN V EN TOR. VERN ER D. COOPIBS ATTORNEY United StatesPatent 3,264,865 FATIGUE TESTING SHAKE TABLE APPARATUS Verner D. Coombs,Erie, Pa, assignor to General Electric Company, a corporation of NewYork Original application Mar. 30, 1962, Ser. No. 183,787,

now Patent No. 3,137,566, dated June 8, 1965. Divided and thisapplication Mar. 9, 1965, Ser. No. 438,-

5 (Ilaims. (Cl. 73-716) The present invention relate to a fatigue andWear testing apparatus, and more particularly to such an apparatus fortesting specimens under cyclic loads. This application is a division ofco-pending application Serial No. 183,787, filed March 30, 1962, now US.Patent No. 3,187,566.

A complete analysis or description of the fatigue behavior of metals ormaterials normally requires extensive fatigue data since there arevarious influences that can be placed on an individual metal ormaterial. Conventional fatigue testing machines that can developcomprehensive test data on specimens present recognized limitationsincluding time of testing and limitations on the number of specimensthat can concurrently be tested. The total expenditures for a fatiguetest using conventional machines consist primarily of specimenpreparation costs, costs for mounting the specimen and adjusting theparticular test machine, and the necessary costs for personnel tomonitor the test itself. From the viewpoint of available time, a maximumnumber of test specimens with a minimum number of adjustments,calibrations, or failure detections on individual specimens isdesirable.

Most conventional fatigue testing machines may be placed in one of twomain categories: First, those machines that impose an alternatingdisplacement upon a specimen or specimens, and second, those machinesthat impose an alternating load upon the specimen or specimens.

In order to predetermine an alternating stress in a specimen in thealternating displacement test, it is usually necessary to express thedisplacement in terms of some load that could cause such a displacement,and to calculate the stress from that value of load. When therelationship between the displacement and the equivalent load has beencalculated or measured for a single specimen, it is usually expedient tomake any additional specimens precise copies of the first, by accuratemachining, in order that the load-displacement relationship need not bedetermined for each specimen. However in an alternating load test, it isonly necessary to calculate the stress level for a given load and thenset the testing machine for that particular load. The accuracy of thiscalculated stress level is influenced by the geometry of the specimenonly in the failure region, and thus accurate machining of the specimenis important only in the expected failure region, for the alternatingload test. It is also possible in an alternating displacement test for acrack to initiate in the specimen but not to develop immediately tocomplete failure; while in an alternating load test, initiation of acrack in a test specimen is generally followed immediately by a failureof the specimen.

Some examples of conventional fatigue testing machines of thealternating displacement type would include those where a stationaryspecimen is deflected by a cam or crank arm action, or a rotatingspecimen is suitably deflected, or a stationary specimen is excited atits natural frequency. Some examples of alternating load testingmachines may include those where a stationary specimen is loaded by aspring that is deflected by a cam or an eccentric action, or where astationary specimen is loaded by a screw mechanism or by a hydraulicmechanism that is load sensed and controlled. Other examples ofalternating load testing machines are where Patented August 9, 1966 arotating specimen is loaded by Weights or by springs, or a stationaryspecimen is loaded by a rotating unbalance, and where a vibratingcantilever beam type specimen has a mass attached.

The aforementioned types of known fatigue testing machines include mostof those that have found specific use in the past. Each machine usuallyhas certain advantages and disadvantages peculiar to its individualdesign. Generally, the particular type of machine achieves a desiredeffect only by sacrificing a desirable flexibility of application.

in view of the known limitations of prior art fatigue testing machines,this invention contributes a fatigue testing machine having flexibilityto provide a capability for testing a wide variety of shapes and sizesof test specimens as well as more flexible modes of loading thesespecimens.

Accordingly, it is a primary object of the invention to provide a newand improved fatigue testing machine of the constant load amplitudetype.

Another object of the invention is to provide a new and improved fatiguetesting machine that facilitates mounting and testing of one or morevarieties of specimens at high cyclic rates without interaction betweenthe specimens.

A further object of the invention is to provide a new and improvedfatigue testing machine that can accommodate specimens which requirelittle accurate machining and no previous calibration.

Still a further object of the invention is to provide a new and improvedfatigue testing machine that is adapted to mount multiple specimens andoperate under elevated temperature and mean-load conditions.

Yet another object of the invention is to provide a new and improvedfatigue testing machine that will facilitate imposing a variety ofstress levels in Various sequences on a specimen.

An additional object of the invention is to provide a new and improvedfatigue testing machine that requires no special failure sensing devicefor a specimen.

A further object of the invention is to provide a new and improvedfatigue testing machine that does not require load sensing and controlof amplitude of displacement.

Briefly stated, the invention in one form thereof comprises a shaketable having a test specimen positioned thereon to develop complexmotions in synchronism with the table. A mass is coupled to the testspecimen by a support means that imparts cyclic rotation from the tableto the mass and thus develops complex fatigue testing stresses on thetest specimen.

The novel features of the invention are pointed out with particularityin the claims appended to and forming part of this specification.However, the organization and operation, together with further objectsand advantages of the invention may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a perspective view of a fatigue testing machine formed inaccordance with the invention and having one form of test specimenarrangement secured thereto;

FIG. 2 is an enlarged plan view of a test specimen secured to a fatiguetester such as shown by FIG. 1;

FIG. 3 is an alternate form of test specimen arrangement in accordancewith the invention;

FIG. 4 is a perspective view of a test specimen of FIG. 2 with a cyclicload positioned thereon;

FIG. 5 is a stress vector pattern exerted on the test specimen shown byFIG. 4, and taken along section 55 of FIG. 4;

FIG. 6 is a side view of the test specimen of FIG. 4 under cyclic loadconditions with the dash line position indicating a maximum deflectionof the test specimen during operation of the fatigue tester of theinvention; and

FIGS. 7 and 8 illustrate partial perspective views of the forms of thetesting apparatus of this divisional application.

As described in the above-identified application of which this is adivision, the testing apparatus of FIG. 1 comprises one form of aconventional shake table having a top member 10 that is supported atopposite ends by flexible end pieces 11 and 1 2. End pieces 11 and 1 2are secured to a support means such as the platform '14 to space the topmember 10 from the platform 14 and to permit cyclic motion of the topmember 1a). Cyclic motion is imparted to the top member 16 by any knownarrangement, for example, the conventional rotating wheel and crank armlinkage arrangement '15 as shown by FIG. 1. The wheel may be rotated atvariable speeds as hereinafter explained. It is contemplated that theshaft 16 is driven by a controllable speed motor, not shown.

The desired fatigue test data is obtained from the fatigue testingmachine of the invention by a plurality of test specimens 17 that areremovably secured to the top member v10 by any known means, for example,bolts or the like. One or more test specimens 17 may be secured to thetop member 10 for testing at one time. In one possible arrangement ofthe test specimens, the specimens '17 extend horizontally in acantilever beamtype arrangement and are necessarily positioned so thatthe direction of motion of the top member 1t) is across the expectedaxes of rotation of rings 34 and 35, hereinafter described, these wouldordinarily be the beam axes of specimens :17. When this condition issatisfied, in the fatigue tester of the invention, the alignment of thetest specimens 17 with regard to the top member is not critical.

A single test specimen 17 is more clearly shown by FIG. 2. The testspecimen 17 is removably secured to the top member 10 of a shake tableto extend in a cantilever fashion as described. The test specimen 17should have a necked portion 20 to act as a region of stress levelhigher than in other regions of the test specimen 17, in order to locatethe expected region of failure so that the failure stress level may beestimated and preset for the test. The test specimen 17 also has areduce-d portion such as a race or groove 21 formed by opposed notches22 and 23. The race or groove 21 is generally adjacent the free end ofthe cantilevered test specimen 17.

Another form of test specimen that may be removably secured to thefatigue tester of the invention is shown by FIG. 3. A readily producedtest specimen 24 is removably secured to the top member 10 of the shaketable, previously described and illustrated by FIG. 1, in a cantileverfashion. The test specimen 24 has a necked portion 20 to control theregion of fatigue during a test. In lieu of a separate race or groovebeing formed in the test specimen 24, an appendage 28 is removablysecured to the free end of the cantilevered test specimen 24 to extendalong the beam axis. The appendage 28 provides a reusable element forvarious test specimens and accordingly can be machined to more criticaltolerances. The appendage 28 has a reduced portion such as a race orgroove 29 that may be formed by opposing notches 3t) and 31, or may bemachined from blank stock to provide something other than a rectangularcross-section at the reduced portion.

The particular test specimens, either specimen 17 as illustrated byFIGS. 1 and 2, or test specimen 24 as illustrated by FIG. 3, arecyclically loaded, as illustrated by FIG. 4. The test specimen 17 isloaded at the race or groove 21 by a revolving mass such as ring 32. Thetest specimen 17 is removably secured to the top member 10,

partially shown, as previously described and illustrated by FIG. 1. Thetest specimen 17 has a necked portion 20 to limit the region of fatigueduring the test and is secured to the top member 19 so that the axis ofrotation of ring 32, and the beam axis of the specimen are across atleast one direction of motion of the top member 10 of the shake table.The inner circumferential surface 33 of the ring 32 impinges upon therace or groove 21 as the ring 3-2 is caused to rotate in synchronismwith the motion of the shake table transmitted from the top member 16through the test specimen 17. The frequency of the shake table is easilyvaried to facilitate starting and accelerating the ring 32 to operatingspeed.

As is readily apparent in FIGS. 1, 4, and 6 the mass, such as the rings32, 34, and 35, is coupled to the test specimen so that its center ofmass is displaced from the beam axis of the test specimen and it is alsofree to revolve. Thus the .motion of the test specimen excites the massto revolve about an axis of the test specimen as well as about an axisof the mass. Accordingly, the fatigue tester of the invention cycliclyloads each test specimen 17 by a centrifugal force vector P, asillustrated by FIG. 4. The magnitude of the force vector F when therevolving mass such as the ring 32 acts upon the test specimen 17 can bedetermined by the following formula:

The centrifugal force vector F equals mw r where This equation assumesthat the path in space of the ring center is circular; tabledisplacement and specimen deflection distort that path; however, anyerror introduced by this assumption is minor and easily compensated for.

In a flat cantilever beametype test specimen, such as test specimen 17,illustrated by FIGS. 1, 2 and 4, the action of the force vector F on thenecked portion 20 is more particularly shown by FIGS. 5 and 6. Thewhirling ring 32 cyclically loads the test specimen 17 from a maximumtensile stress to a maximum compressive stress during each cycle ofrevolution of the ring 32. The maximum limits of stress are shown byFIG. 5 Where the stress vector S exhibits maximum tensile andcompressive stresses at points A, A during each revolution of the ring32. 'FIG. 5 illustrates a sectional view through the necked portion 20of the test specimen '17 as illustrated by FIGS. 2 and 4. The stressvector S acts upon an imaginary point 44 as a result of the deflectionof the test specimen 17 under the cyclic load of the ring 32, as bestshown by FIG. 6. In FIG. 6, the maximum deflection is shown by thedashed line position of the ring 32 and the centilever beam specimen 17under the loading of the force vector F. The stresses on the flatsurface of the necked portion 20 of the test specimen 17 are thereforevaried cyclicly and not necessarily sinusoidally from a maximum tensileto a maximum compressive during each cycle of revolution of the ring 32.

The peak value of stress, whether tensile or compressive, S for eachtest specimen is:

where Thus,

a m'w rl The important aspect of the stress Equation III is that thepeak value of stress S imposed on a test specimen is dependent upon: (1)the frequency at which the table oscillates, (2) the physicalcharacteristics of the ring, (3) the thickness and width at the neck ofthe specimen, and (4) the neck-t-o-groove distance. This means that thestress amplitude applied to each test specimen is very accuratelymaintained in substantially automatic fashion. The accuracy of test datafor each specimen is not influenced to any significant degree by eitherthe action of adjacent test specimens and rings, the amount of cyclicdeflection the table assumes, or the precision with which the specimenis mounted.

Referring again to FIG. 1, it is illustrated therein that the diametersof the ring, such as ring 32 illustrated by FIG. 4, can vary to imposedifferent cyclic load factors on particular test specimens. The mass ofeach ring can also be varied as desired. Ring 34 has a dimension greaterthan that of ring 35 so that the cyclic load conditions imposed upon thetest specimens 17 as illustrated by FIG. 1 will differ since theresulting centrifugal force vectors will have different magnitudes. Thecentrifugal force may also be varied by varying the frequency of theshake table, and thus the frequency of rotation of the rings 32, 34, 35.Variable frequency of the shake table as illustrated in FIG. 1 may beobtained by varying the speed at which shaft 16 is driven.

Thus, the fatigue testing arrangement of the invention reduces the costof fatigue testing, -as a plurality of test specimens can be testedwithout conflicting actions on a conventional shake table. The specimenscan be quickly secured in place on the shake table without requiringdirect power loading of each test specimen. Ample test data can beobtained for a given type specimen and for a given cost by the fatiguetester of the invention. Further, the test arrangement of the fatiguetesting machine of the invention offers versatility as to the varioustypes of conditions that may be imposed upon each test specimen. Theeffect of gravity may be added to the parameters of the stress equationto produce mean stress influences. The factor of temperature may also beintroduced by encompassing the fatigue testing machine as illustrated byFIG. 1 within an oven or the like. Additionally, failure in a testspecimen of a ductile material is not ordinarily catatrophic and at theearlier stages of a crack propagation in the necked portion, therevolving ring will drop out of synchronism With the tape and stop, thusproviding a built-in indicator device for an operator of a fatiguetesting arrangement embodying the invention.

The shake table may take other forms than that illustrated in FIG. 1.For example, a shake table has been constructed wherein the top memberis formed of a plurality of rigid bars supported at either ends thereofby members adapted to flex when cyclic movement is imparted to the shaketable. Also, the specimens may be secured to the shake table at one endthereof and depend vertically therefrom, with the loading rings or hoopsspinning in a horizontal plane. The term cantilever is intended to coverthis type of arrangement as well as the arrangement of FIG. 1.

In FIG. 7 there is shown one embodiment of the invention of thisdivisional application. As shown, the testing apparatus provides for thetesting of bearings, such as radial bearings and the like. To this end,the radial bearing 36 is positioned on a journal 37 that is fixed to thetop member 10 of a shake table, as previously described and illustratedby FIG. 1. The radial bearing 36 is retained by a bearing holder 38 thathas a radially extending revolving mass 39 at a predetermined distancefrom the journal 37. The test arrangement as illustrated by FIG. 7obtains wear life data on bearings under combined speed and loadconditions. The rotating mass 39 provides a centrifugal force similar tothe centrifugal force vector and stress vector previously discussed andillustrated by FIGS. 4 and 5.

Fatigue producing loads are provided by an additional modification ofthe testing apparatus of the invention as illustrated by FIG. 8. Apreformed cup 40 is either secured to the top member 10 of aconventional shake table by a suitable means such as welding, or the cup40 may be formed integral with the top member HE. A bearing, such asradial bearing 41, is retained within the cup 40. A revolving mass 42 isfixed to a journal 43 at a predetermined radial distance and excited torotate by the response of the mass 42 to the cyclic action of the topmember If) of the shake table.

As shown in FIGS. 7 and 8, the mass is coupled to the bearing so thatits center of mass is displaced from the axis of rotation of thebearing. When motion is imparted to the bearing from the platform, themass is caused to revolve about the axis of rotation of the bearingthereby setting up the desired wear or fatigue stresses on the bearing.

The testing apparatus of the invention as previously described providesa low cost device usable with low cost test specimens. It furtherprovides an arrangement whereby numerous specimens may be testedsimultaneously without conflict or interaction between adjacentspecimens during the test.

As will be evidenced from the foregoing description, certain aspects ofthe invention are not limited to the particular details of theconstructions as illustrated. For example, the fatigue testing of gearassemblies or individual gears under load conditions is possible by theimproved fatigue testing arrangement of the invention. It iscontemplated that other embodiments, modifications and applications ofthe invention may occur to those skilled in the art, and it is thereforeintended that the appended claims shall cover all such embodiments,modifications and applications that do not depart from the spirit andscope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Testing apparatus comprising: a platform adapted to support one ormore cylindrical journal means each adapted to retain a bearing thereon;means for oscillating said platform; means securing said journal meansto said platform so that the cylindrical axis of said journal means isperpendicular to the plane containing the directions of motion of saidplatform whereby motion is developed in said journal means and thebearing retained thereby which motion is in synchronism with saidplatform; a bearing holder adapted to removably engage said bearing; andmeans for imposing a cyclic force on said bearing, said means includinga mass revolving about the cylindrical axis of said journal means, saidmass being coupled to said bearing holder so that its center of mass isdisplaced from the cylindrical axis of said journal means and saidbearing holder is caused to revolve about the axis of said journal meansby the motion imparted to said journal means by said platform.

2. Testing apparatus comprising: a shake table adapted to have cyclicmovement imparted thereto; cylindrical journal means fixed to said tableso that the cylindrical axis of said journal means is perpendicular tothe plane containing the directions of motion of said table whereby abearing retained by said journal means has motions developed therein insynchronism with movement of said table; a bearing holder adapted toremovably engage a bearing retained by said journal means, said bearingholder having a mass associated therewith which mass is radiallydisplaced from the cylindrical axis of said journal means and said massrevolves about the cylindrical axis of said journal means in rotationalsynchronism with movement of said table to develop a cyclic force andresultant wear stresses on said bearing.

3. Testing apparatus comprising: a shake table adapted .9.. ave. cyclicmotion imparted thereto; a hollow cylindrical bearing holder fixed tosaid table so that the cylindrical axis of a bearing retained by saidbearing holder is perpendicular to the plane containing the directionsof motion of said table to develop motions in such bearing insynchronism with movement of said table; a journal means adapted toremovably engage a bearing retained within said bearing holder, saidjournal means having a mass associated therewith which mass is radiallydisplaced from the cylindrical axis of said bearing so that motion ofsaid bearing excites said bearing holder to revolve about thecylindrical axis of said bearing and develop a cylic force and resultingfatigue stresses on said bearing.

4. Testing apparatus comprising: a platform, means for imparting cyclicmotion to said platform; a hollow cylindrical bearing holder mounted onsaid platform and having its cylindrical axis perpendicular to the planecontaining the directions of motion of said platform so that motion insynchronism with said platform is imparted to a bearing retained by saidbearing holder; journal means adapted to removably engage said bearing;means for developing a cyclic force about the axis of rotation of saidbearing, said means including a mass revolving about the axis ofrotation of said bearing and being coupled to said bearing so that itscenter of mass is displaced from the axis of rotation of said bearing.

5. Testing apparatus comprising: a platform; means for imparting cyclicmotion to said platform; a hollow cylindrical bearing holder mounted onsaid platform and having its cylindrical axis perpendicular to the planecontaining the directions of motion of said platform so that motion insynchronism with said platform is imparted to a bearing retained by saidbearing holder; journal means adapted to removably engage said bearing;and a mass coupled to said journal means so that its center of mass isdisplaced from the axis of rotation of said bearing so that the motionof said bearing causes said mass to revolve about the axis of rotationof said bearing to develop a cyclic force thereabout and resultantfatigue testing stresses thereon.

References Cited by the Examiner UNITED STATES PATENTS 2,439,035 4/1948Bidwell et al 737 RICHARD C. QUEISSER, Primary Examiner.

G. M. GRON, Assistant Examiner.

2. TESTING APPARATUS COMPRISING: A SHAKE TABLE ADAPTED TO HAVE A CYCLICMOVEMENT IMPARTED THERETO; CYLINDRICAL JOURNAL MEANS FIXED TO SAID TABLESO THAT THE CYLINDRICAL AXIS OF SAID JOURNAL MEANS IS PERPENDICUALR TOTHE PLANE CONTAINING THE DIRECTIONS OF MOTION OF SAID TABLE WHEREBY ABEARING RETAINED BY SAID JOURNAL MEANS HAS MOTIONS DEVELOPED THEREINSYNCHRONISM WITH MOVEMENT OF SAID TABLE; A BEARING HOLDER ADAPTED TOREMOVABLY ENGAGE A BEARING RETAINED BY SAID JOURNAL MEANS, SAID BEARINGHOLDER HAVING A MASS ASSOCIATED THEREWITH WHICH MASS IS RADIALLYDISPLACED FROM THE CYLINDRICAL AXIS OF SAID JOURNAL